A number of hazardous compounds are found in the off-gas- and/or exhaust emissions of chemical, fuel-consuming and industrial processes. These may originate from industrial exhausts, and off-gas emissions from chemical plants, including purified terephthalic acid (PTA) plants. The hazardous compounds found in various kinds of emissions may be in the form of chemical by-products and solvents. Likewise, the hazardous compounds—including halogenated VOCs and other highly toxic pollutants—pose a number of deleterious effects to people and the environment, including respiratory problems, various cancers, smog, and ozone depletion. Further, using incineration to eliminate some of these compounds, such as certain halogenated compounds, might produce other toxic substances depending on the operating conditions. For these and other reasons involving public health and environmental considerations, local, regional or global regulations have required the elimination of these hazardous compounds.
Accordingly, the hazardous compounds contained in certain emissions, which the catalysts and processes of the current embodiments can be used to eliminate, generally include volatile organic compounds (“VOC”), halogenated volatile organic compounds (“HVOC”), and carbon monoxide (“CO”). In many cases, though not all cases, such compounds contain carbon and hydrogen, and may also contain other elements such as oxygen, nitrogen and halogen.
To date, various approaches have been used for eliminating these hazardous compounds, e.g., adsorption on carbon or other adsorbents, catalytic oxidation, and thermal oxidation (incineration). However, adsorption only concentrates the hazardous pollutants, without destroying them, and the effectiveness of the approach depends on fluctuations in concentration of the pollutants. Moreover, thermal oxidation is marked by high operating temperatures and substantial operating costs because these reactions require high temperatures—often exceeding 750° C. By comparison, catalytic oxidation reduces the activation energy needed to oxidize these hazardous compounds in the presence of oxygen, so it can be accomplished at a fraction of the energy required for thermal oxidation.
For brevity, compounds that can be destroyed with the inventive catalysts and processes disclosed herein are referred to as the “hazardous compounds.” These include, but are not necessarily limited to carbon monoxide; VOCs such as benzene, toluene, xylene, methanol, and methyl acetate; and HVOCs such as chloromethane, dichloromethane, dichloroethane, bromomethane, dibromomethane, dibromoethane, and chlorobenzene, dichlorobenzene, and polybromobenzene, to name several. The inventive catalysts are used to reduce the activation energy needed for these reactions to occur, and are generally provided as a low-cost approach compared to other catalysts used for oxidation.
In general, catalysts that have been utilized for the oxidation reactions involved with destruction of the hazardous compounds have included platinum, palladium and rhodium on aluminum oxide (Al2O3) supports. Such catalysts have taken various forms, such as tablets, pellets, spheres, and granules, as well as monolithic supports.
Because catalytic oxidation reactions are carried out at lower temperatures than thermal oxidation, it reduces operating costs, and provides other significant advantages for operations, including economic savings. But although catalytic oxidation is considered a viable approach, the use of expensive precious metal catalysts such as platinum and palladium limit the economic savings, and there is a desire for low cost catalyst having similar effectiveness. Further, catalysts that have been used previously for these purposes sometimes cost more, so there is a need for catalysts that provide a longer service life and lower the costs.